In recent years, a side airbag for protecting the heads of passengers when the vehicle collides is mounted on the seat back side portion or the roof portion of the compartment of a vehicle. This side airbag unit includes an inflator for generating inflation gas after a large shock is received due to a vehicle collision and an airbag, which is inflated to expand with the gas supplied from the inflator. The airbag, when stored before the side airbag unit is activated, is folded in a predetermined procedure and stored in the seat back side portion of a vehicle seat or the like, while the side airbag unit itself is covered with a seat cover or the like.
Usually, the surface of the vehicle seat is covered with a seat cover made of fabric or leather for covering the cushion material of the seat and an inflation-out opening to allow the airbag to inflate is formed on a portion of the seat cover on the seat bag side portion where the side airbag unit is provided. Conventionally, this inflation-out opening is sewed together with a brittle sewing thread so that it is cut when pulled by a predetermined force.
This side airbag unit senses a shock when receiving a large shock due to a collision of a vehicle to generate a high pressure gas in the inflator and introduces the gas into the airbag to inflate the airbag at once. When the airbag is inflated, a large pulling force is applied to the sewing thread which sews the inflation-out opening so that the sewing thread is cut by this force so as to open the closed inflation-out opening and then the airbag is inflated out from the opening.
This airbag is expanded sideway of a passenger to support the head, chest and waist of the passenger by absorbing a shock. Consequently, the shock applied to the human body at the time of a collision is cushioned largely to secure the safety of the passengers.
However if a structure for closing the airbag inflation-out opening using the sewing thread is adopted, the manufacturing procedure is limited to have a step of putting the seat cover after the airbag unit is mounted on the seat, which is inconvenient. For a recent vehicle seat, a flexible manufacturing procedure is demanded since diversified auxiliary machines such as a heater, a seat level adjusting actuator and the like are mounted in many cases. Also conventionally, if inspection of the airbag unit is requested after a vehicle is delivered to a customer, the airbag unit can be inspected only after the seat cover is removed, which complicates the working procedure.
Further, the rupture strength of the sewing thread varies largely depending on sewed condition and further, varies largely with a rise of temperature in a room and deterioration due to aging, which cause an inconvenience. If the sewing thread is not likely to be cut out, a serious fault such that the airbag is not inflated out on emergency is caused. If the strength of the sewing thread is low, the sewing thread is cut by a pulling force applied on the seat cover when a passenger sits on the seat resulting in that the inflation-out opening is cloven to expose the seat cushion.
When the airbag is inflated to expand, the sewing thread is cut one by one sequentially and thus, the expansion speed of the airbag drops, which is another inconvenience.
Instead of the structure for closing the inflation-out opening using the sewing thread having such various inconveniences, a closing structure of the inflation-out opening using the slide fastener has attracted attention. By using the slide fastener, the inflation-out opening can be opened and closed by moving the slider up and down even after the slide fastener is sewed on a seat cover. Thus, the flexibility of assembling procedure is intensified thereby facilitating the assembly of a seat having a complicated structure. Further, upon inspection of the airbag unit, its wirings and attaching portion can be inspected easily by opening and closing the slide fastener.
When coupling of coupling teeth in part of a slide fastener in which the coupling teeth are coupled is released and a lateral pulling force (a force per unit length of coupling axis to be applied in a direction for separating the coupling teeth attached to a pair of fastener tapes, right and left) is applied continuously, the coupling of the coupling teeth can be released sequentially with a weak force without moving the slider. Thus, the expansion speed can be accelerated unlikely to a situation of cutting the sewing thread one by one. Accordingly, a slide fastener is preferable for quickly expanding the airbag to protect the passenger effectively. Particularly, because a gap between the head of the passenger and the side glass of a vehicle is small, application of this fastener to a side airbag, which needs to be inflated to expand in a short time after a collision, is expected.
For example, a Patent Document 1 (Japanese Patent Application Laid-Open No. 2006-15158) discloses a slide fastener with an emergency opening means having a structure, which cleaves easily, for use on the inflation-out opening of the airbag.
The coupling tooth of the slide fastener with an emergency opening means described in the Patent Document 1 has a shape including: two leg portions fixed to the fastener tape side edge striding over the coupling tooth attaching portion of the fastener tape side edge; a body portion connecting the two leg portions; a coupling head formed at an external end in order to couple a pair of the coupling teeth on right and left in a lateral pulling force direction; and a neck portion which is constricted and for connecting the coupling head with the body portion. A groove is formed in the summit of the coupling head along a coupling axis. Further, a shoulder portion to be fitted to the groove is formed in the central portion in the width direction of the coupling tooth such that it is projected from the body portion and the neck portion.
The coupling head is fitted into between the neck portions of two adjacent coupling teeth fixed to the other fastener tape disposed to oppose in order to prevent the coupled coupling teeth from being cloven in the lateral pulling force direction. The shoulder portion formed such that it is projected from the neck portion and the body portion of the coupling tooth is fitted to the groove portion provided in the opposed coupling head, thereby preventing right and left fastener stringers from being deviated in a shearing direction which may release the coupling.
In the slide fastener described in the Patent Document 1, two coupling teeth formed in a nose shape having asymmetrical shape in its front and rear sides by cutting out the rear face of the coupling teeth, which are fitted into the shoulder portion of a coupling tooth, are disposed in the central portion of the slide fastener. Such configured portion is easily cloven by a pushing up force from the rear face side of the slide fastener, and thus it serves as a cleavage starting point in the slide fastener when the airbag is inflated out.
Further, a Patent Document 2 (Japanese Patent Application Laid-Open No. 2004-298641) has disclosed a slide fastener having a holding means which serves as a cleavage starting point in order to facilitate the slider fastener to open from the central portion of the slide fastener when an internal pressure is applied by an airbag.
The holding means to be the cleavage starting point in the central portion of the slide fastener disclosed in the Patent Document 2 may be a holding means having a constricted portion of synthetic resin which is broken by an expansion force when the airbag is inflated (see FIG. 1 in the Patent Document 2), a holding means using a welded tape which is broken by an expansion force when the airbag is inflated (see FIG. 3 in the Patent Document 2), a holding means in which a male type engaging head is previously inserted into a engaging hole in the central portions in right and left fastener tapes and the engaging heads are released from the engaging hole by the expansion force when the airbag is inflated (see FIG. 4 in the Patent Document 2), or the like.    Patent Document 1: Japanese Patent Application Laid-Open No. 2006-15158    Patent Document 2: Japanese Patent Application Laid-Open No. 2004-298641